Short-term changes in polysaccharide utilization mechanisms of marine bacterioplankton during a spring phytoplankton bloom

Spring phytoplankton blooms in temperate environments contribute disproportionately to global marine productivity. Bloom-derived organic matter, much of it occurring as polysaccharides, fuels biogeochemical cycles driven by interacting autotrophic and heterotrophic communities. We tracked changes in the mode of polysaccharide utilization by heterotrophic bacteria during the course of a diatom-dominated bloom in the German Bight, North Sea. Polysaccharides can be taken up in a ‘selfish’ mode, where initial hydrolysis is coupled to transport into the periplasm, such that little to no low-molecular weight (LMW) products are externally released to the environment. Alternatively, polysaccharides hydrolyzed by cell-surface attached or free extracellular enzymes (external hydrolysis) yield LMW products available to the wider bacterioplankton community. In the early bloom phase, selfish activity was accompanied by low extracellular hydrolysis rates of a few polysaccharides. As the bloom progressed, selfish uptake increased markedly, and external hydrolysis rates increased, but only for a limited range of substrates. The late bloom phase was characterized by high external hydrolysis rates of a broad range of polysaccharides and reduced selfish uptake of polysaccharides, except for laminarin. Substrate utilization mode is related both to substrate structural complexity and to the bloom-stage dependent composition of the heterotrophic bacterial community.     

Nutrient control of bacterioplankton and phytoplankton dynamics

To determine whether positive correlations between phytoplankton and bacterioplankton growth in nutrient addition experiments are due to growth coupling or growth stimulation by the same nutrients, we examined phyto- and bacterioplankton growth in a series of eleven nutrient addition (N × P) and light/dark experiments. In mesotrophic Castle Lake, the phyto- and bacterioplankton growth responses to phosphorus (P) addition were strongly correlated (r²=0.59), while only a weak correlation (r²=0.10) was observed for the nitrogen addition treatments. After normalizing the N + P treatments for the growth stimulation observed in the respective P treatments, we found a substantial stimulation of the phytoplankton (e.g., costimulation by N + P) and no stimulation of the bacterioplankton. Bacteria growth rates were similar in both light and dark incubated P treatments. In these experiments, we found clear evidence suggesting the dynamics of bacteria and phytoplankton were correlated because they are often limited by the same resource (mainly inorganic phosphorus). We found only limited evidence that bacterioplankton growth coupling to algal dynamics was occurring in these experiments. However, we did not consider several factors such as dissolved organic nutrient availability, bacterivory, availability of physical substrates, and temperature which are also thought to influence the nature of bacterial/phytoplankton interactions. Based on the results of our experiments, we conclude the biomass of the bacterio- and phytoplankton covaried because they were stimulated by the same nutrients. This revised version was published online in August 2006 with corrections to the Cover Date.     

Linking the composition of bacterioplankton to rapid turnover of dissolved dimethylsulphoniopropionate in an algal bloom in the North Sea

The algal osmolyte, dimethylsulphoniopropionate (DMSP), is abundant in the surface oceans and is the major precursor of dimethyl sulphide (DMS), a gas involved in global climate regulation. Here, we report results from an in situ Lagrangian study that suggests a link between the microbially driven fluxes of dissolved DMSP (DMSPd) and specific members of the bacterioplankton community in a North Sea coccolithophore bloom. The bacterial population in the bloom was dominated by a single species related to the genus Roseobacter , which accounted for 24% of the bacterioplankton numbers and up to 50% of the biomass. The abundance of the Roseobacter cells showed significant paired correlation with DMSPd consumption and bacterioplankton production, whereas abundances of other bacteria did not. Consumed DMSPd (28 nM day⁻¹) contributed 95% of the sulphur and up to 15% of the carbon demand of the total bacterial populations, suggesting the importance of DMSP as a substrate for the Roseobacter -dominated bacterioplankton. In dominating DMSPd flux, the Roseobacter species may exert a major control on DMS production. DMSPd turnover rate was 10 times that of DMS (2.7 nM day⁻¹), indicating that DMSPd was probably the major source of DMS, but that most of the DMSPd was metabolized without DMS production. Our study suggests that single species of bacterioplankton may at times be important in metabolizing DMSP and regulating the generation of DMS in the sea.     

Diversity and biomass dynamics of unicellular marine fungi during a spring phytoplankton bloom

Microbial communities have important functions during spring phytoplankton blooms, regulating bloom dynamics and processing organic matter. Despite extensive research into such processes, an in-depth assessment of the fungal component is missing, especially for the smaller size fractions. We investigated the dynamics of unicellular mycoplankton during a spring phytoplankton bloom in the North Sea by 18S rRNA gene tag sequencing and a modified CARD-FISH protocol. Visualization and enumeration of dominant taxa revealed unique cell count patterns that varied considerably over short time scales. The Rozellomycota sensu lato (s.l.) reached a maximum of 10⁵ cells L⁻¹, being comparable to freshwater counts. The abundance of Dikarya surpassed previous values by two orders of magnitude (10⁵ cells L⁻¹) and the corresponding biomass (maximum of 8.9 mg C m⁻³) was comparable to one reported for filamentous fungi with assigned ecological importance. Our results show that unicellular fungi are an abundant and, based on high cellular ribosome content and fast dynamics, active part of coastal microbial communities. The known ecology of the visualized taxa and the observed dynamics suggest the existence of different ecological niches that link primary and secondary food chains, highlighting the importance of unicellular fungi in food web structures and carbon transfer.     

Colloidal organic carbon and phytoplankton speciation during a coastal bloom

During the 1995 spring bloom in Bedford Basin, Nova Scotia,dissolved organic carbon (DOC) in the photic zone was separatedinto low-molecular-weight and colloidal size fractions by cross-flowultrafiltration. DOC, colloidal organic carbon (COC) and low-molecular-weightorganic carbon (LOC) were then analyzed by high-temperaturecatalytic oxidation. COC (associated with the production ofphytoplankton exudates) did not follow the concentration ofbulk chlorophyll a (chi a) or the total number of phytoplanktoncells. Instead, surface-active COC (that adhered to the ultrafiltrationsystem) was produced early during the bloom when the diatomSkeletonema costatum was at a maximum. Later on, as the bloombegan to decline, less surface-active COC (that remained largelyfree of the ultrafiltration surfaces) was produced and was associatedmore with variations in cell number of Chaetoceros socialis,the predominant diatom for most of the bloom. These resultssuggest that chl a or the total number of phytoplankton cellsmay not be reliable indices of the production of COC. On thecontrary, the results suggest that relatively high COC concentrationswere associated with specific diatom species. In addition, themaximum amount of COC was associated during the early stagesof the bloom with a diatom (S.costatum) that was a small fraction(<5.6%) of total phytoplankton cell number. This finding,that relatively large amounts of colloids were associated withexopolymer production during the onset and development of abloom, does not agree with reports suggesting that the productionof exopolymers by diatoms is primarily an end-of-bloom occurrence.     

Dinoflagellate-cyanobacterium communication may determine the composition of phytoplankton assemblage in a mesotrophic lake

The reasons for annual variability in the composition of phytoplankton assemblages are poorly understood but may include competition for resources and allelopathic interactions. We show that domination by the patch-forming dinoflagellate, Peridinium gatunense, or, alternatively, a bloom of a toxic cyanobacterium, Microcystis sp., in the Sea of Galilee may be accounted for by mutual density-dependent allelopathic interactions. Over the last 11 years, the abundance of these species in the lake displayed strong negative correlation. Laboratory experiments showed reciprocal, density-dependent, but nutrient-independent, inhibition of growth. Application of spent P. gatunense medium induced sedimentation and, subsequently, massive lysis of Microcystis cells within 24 hr, and sedimentation and lysis were concomitant with a large rise in the level of McyB, which is involved in toxin biosynthesis by Microcystis. P. gatunense responded to the presence of Microcystis by a species-specific pathway that involved a biphasic oxidative burst and activation of certain protein kinases. Blocking this recognition by MAP-kinase inhibitors abolished the biphasic oxidative burst and affected the fate (death or cell division) of the P. gatunense cells. We propose that patchy growth habits may confer enhanced defense capabilities, providing ecological advantages that compensate for the aggravated limitation of resources in the patch. Cross-talk via allelochemicals may explain the phytoplankton assemblage in the Sea of Galilee.     

太湖浮游细菌与春末浮游藻类群落结构演替的相关分析

为研究浮游细菌与浮游藻类群落演替的相关性,2005年4月至6月在太湖5个观测点采集浮游细菌及浮游藻类样本。分别采用聚合酶链式反应-变性梯度凝胶电泳（PCR—DGGE）和显微观察的方法分析浮游细菌及浮游藻类群落组成。结果表明,春末夏初,浮游细菌与藻类均呈现较高的多样性,浮游细菌DGGE图谱具有43种不同条带,浮游藻类的常见种有29种。浮游细菌群落聚类分析显示,丝藻（Ulothrix sp．）和微囊藻（Microcystis spp．）占优势时,浮游细菌群落基因组成存在明显差异。以藻类种群Shannon—Wiener多样性指数（Hp）,浮游藻类总细胞数（N）以及Microcystis spp．（M）百分含量为变量,典型对应分析（CCA）结果显示浮游细菌与浮游藻类群落结构变化的相关系数为30．9％,表明春末夏初太湖浮游细菌与浮游藻类群落演替具有较高的相关性。     

North Atlantic Oscillation and spring bloom phytoplankton composition in the English Channel

The spring phytoplankton composition has been investigated ata 50 m deep station off Plymouth in the English Channel for6 years (1993–1999). The percentage of diatoms duringthe spring bloom was significantly correlated with the NorthAtlantic Oscillation index. A similar relationship between phytoplanktonand North Atlantic Oscillation has also been found in a Swedishlake, suggesting a possible link between atmospheric forcingand phytoplankton composition.     

洱海蓝藻水华暴发期浮游植物群落变化及影响因素

对洱海水质和浮游植物进行了调查, 分析了洱海藻类水华暴发季节水质营养水平及浮游植物多样性, 并探讨了洱海浮游植物种群演替的环境机制. 研究结果表明, 洱海蓝藻水华在6-10月份大量暴发, 蓝藻数量高达107 cells/L, 水华种类为微囊藻(Microcystis)属的一些种, 微囊藻在6-10月份占绝对优势, 最高可达90%以上. 分析5-11月洱海营养水平, 其中7月营养水平最高, 浮游植物生物量10月份最大(以叶绿素a表示). 使用SPSS里的pearson分析了浮游植物和各理化因子的关系, 结果表明相对于其他浮游植物种类, 总磷(TP)对微囊藻的影响比较大.       

Relationship between bacterioplankton and phytoplankton community dynamics during late spring and early summer in Lake Taihu, China

The relationship between bacterioplankton and phytoplankton community compositions was investigated from April to June, 2005 with samples taken at five different stations in Lake Taihu, China. Bacterioplankton and phytoplankton community compositions as determined by denaturing gradient gel electrophoresis of polymerase chain reaction amplified bacterial 16S rRNA gene fragments (PCR-DGGE) and microscopic examination, respectively, revealed a high degree of diversity. In total, 43 unique bands were identified amongst the DGGE profiles, and 29 dominant phytoplankton populations were observed. The DGGE profiles were clustered into two distinct groups. These two clusters were closely associated with two dominant phytoplanktons, Ulothrix sp. and Microcystis spp., as observed on April 19, May 27 and June 27, 2005, respectively. The relationship between bacterioplankton and phytoplankton community compositions was investigated using Canonical Correspondence Analysis (CCA) with the Shannon-Wiener diversity index of phytoplankton community (Hp), the total cell abundance of phytoplankton (N) and the relative abundance of Microcystis spp. (M) as the variables. The CCA results indicated a 30.9 % correlation coefficient, suggesting a high degree of correlation between bacterioplankton and phytoplankton community compositions.     

Relationship between bacterioplankton and phytoplankton community dynamics during late spring and early summer in Lake Taihu, China

The relationship between bacterioplankton and phytoplankton community compositions was investigated from April to June, 2005 with samples taken at five different stations in Lake Taihu, China. Bacterioplankton and phytoplankton community compositions as determined by denaturing gradient gel electrophoresis of polymerase chain reaction amplified bacterial 16S rRNA gene fragments (PCR-DGGE) and microscopic examination, respectively, revealed a high degree of diversity. In total, 43 unique bands were identified amongst the DGGE profiles, and 29 dominant phytoplankton populations were observed. The DGGE profiles were clustered into two distinct groups. These two clusters were closely associated with two dominant phytoplanktons, Ulothrix sp. and Microcystis spp., as observed on April 19, May 27 and June 27, 2005, respectively. The relationship between bacterioplankton and phytoplankton community compositions was investigated using Canonical Correspondence Analysis (CCA) with the Shannon-Wiener diversity index of phytoplankton community (Hp), the total cell abundance of phytoplankton (N) and the relative abundance of Microcystis spp. (M) as the variables. The CCA results indicated a 30.9 % correlation coefficient, suggesting a high degree of correlation between bacterioplankton and phytoplankton community compositions.     

Climate change and the timing, magnitude, and composition of the phytoplankton spring bloom

In this article, we show by mesocosm experiments that winter and spring warming will lead to substantial changes in the spring bloom of phytoplankton. The timing of the spring bloom shows only little response to warming as such, while light appears to play a more important role in its initiation. The daily light dose needed for the start of the phytoplankton spring bloom in our experiments agrees well with a recently published critical light intensity found in a field survey of the North Atlantic (around 1.3 mol photons m^?2 day^?1). Experimental temperature elevation had a strong effect on phytoplankton peak biomass (decreasing with temperature), mean cell size (decreasing with temperature) and on the share of microplankton diatoms (decreasing with temperature). All these changes will lead to poorer feeding conditions for copepod zooplankton and, thus, to a less efficient energy transfer from primary to fish production under a warmer climate.     

In situ microcosm experiments on the influence of nitrate and light on phytoplankton community composition

Phytoplankton blooms are fundamental features of coastal ecosystems, but the processes that select for blooms of certain species are not well understood. The aim of this work was to investigate experimentally the interaction of light and nutrients (nitrate) in structuring phytoplankton community composition in Pelorus Sound, New Zealand. Microcosm experiments were conducted in situ nine times throughout the year, providing controls and treatments for increased nutrients and decreased light. Nitrate availability was found to be limiting to phytoplankton growth during spring and summer. Small- to medium-sized, chain-forming diatom taxa such as Chaetoceros sp., Skeletonema sp., Pseudonitzschia sp. and Thalassiosira sp. responded most rapidly to nitrate enrichment, increasing their biovolume up to 32-fold during the 5-day experiments. A long-term phytoplankton monitoring database showed that these taxa have historically dominated the phytoplankton assemblage, suggesting that intense competition for nitrate is a key component in structuring the phytoplankton community. Many of the taxa that were able to withstand light reduction in the shaded treatments were rare historically in Pelorus Sound, suggesting that light is secondary to nitrate availability in structuring the phytoplankton community composition in this coastal embayment.     

Planktonic bacterial community composition of an extremely shallow soda pond during a phytoplankton bloom revealed by cultivation and molecular cloning

Böddi-szék is one of the shallow soda ponds located in the Kiskunság National Park, Hungary. In June 2008, immediately prior to drying out, an extensive algal bloom dominated by a green alga (Oocystis submarina Lagerheim) was observed in the extremely saline and alkaline water of the pond. The aim of the present study was to reveal the phylogenetic diversity of the bacterial communities inhabiting the water of Böddi-szék during the blooming event. Using two different selective media, altogether 110 aerobic bacterial strains were cultivated. According to the sequence analysis of the 16S rRNA gene, most of the strains belonged to alkaliphilic or alkalitolerant and moderately halophilic species of the genera Bacillus and Gracilibacillus (Firmicutes), Algoriphagus and Aquiflexum (Bacteroidetes), Alkalimonas and Halomonas (Gammaproteobacteria). Other strains were closely related to alkaliphilic and phototrophic purple non-sulfur bacteria of the genera Erythrobacter and Rhodobaca (Alphaproteobacteria). Analysis of the 16S rRNA gene-based clone library indicated that most of the total of 157 clone sequences affiliated with the anoxic phototrophic bacterial genera of Rhodobaca and Rhodobacter (Alphaproteobacteria), Ectothiorhodospira (Gammaproteobacteria) and Heliorestis (Firmicutes). Phylotypes related to the phylum Bacteroidetes formed the second most abundant group. Clones related to the mainly anaerobic and alkaliphilic bacterial genera of Anoxynatronum (Firmicutes), Spirochaeta (Spirochaetes) and Desulfonatronum (Deltaproteobacteria) were also abundant. Further clone sequences showed less than 95 % similarity values to cultivated species of the phyla Actinobacteria, Cyanobacteria, Deinococcus-Thermus, Fibrobacteres, Gemmatimonadetes and Lentisphaerae.     

Triggers of phytoplankton bloom dynamics in permanently eutrophic waters of a South African estuary

The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of Heterosigma akashiwo (Raphidophyceae). This study aimed to identify the environmental variables shaping phytoplankton community composition and succession patterns during a typical spring/summer harmful algal bloom (HAB) period. Monitoring of abiotic and phytoplankton variables was undertaken over the period of a month in 2016. Surface water salinity corresponding to mesohaline conditions (9 to 12) was a prerequisite for site selection. During the study, two HABs (>550 µg Chl a l^?1) of H. akashiwo occurred, each lasting for approximately a week in duration. Analyses highlighted nutrient depletion (i.e. nitrate and phosphate concentrations) as the key constraint on bloom duration. When the density of H. akashiwo decreased, the community composition became more diverse with species belonging to Bacillariophyceae and Dinophyceae becoming more abundant; albeit to a lesser degree (<180 µg Chl a l^?1). Dissolved oxygen shifted from super-saturated conditions (>14 mg l^?1) during peak HAB conditions, to instances of bottom water oxygen depletion (2–4 mg l^?1) during the decay phase. These findings highlight the potential severity of transforming a catchment from natural to one that is highly regulated by agricultural practices, while also emphasising the need for management intervention.     

High-throughput cultivation of heterotrophic bacteria during a spring phytoplankton bloom in the North Sea

On-going studies of phytoplankton-bacterioplankton interactions at the long-term ecological research site Helgoland Roads have indicated that many of the heterotrophic bacterial taxa have not yet been cultivated. A high-throughput approach combining whole cell matrix-assisted laser desorption ionization – time of flight mass spectroscopy with 16S rRNA gene sequencing was applied to the spring bloom of 2016. Aiming at an assessment of cultivability during a spring bloom, cultivation on solid marine media had to be used since dilution to extinction would not have been feasible for a high-throughput approach, as performed in this study. A total of 5023 isolates were obtained from nine weekly samples on eight different solid media between the early-bloom and post-bloom periods. Most of the 4136 strains identified affiliated with Bacteroidetes (13.3%), Gammaproteobacteria (26.9%), Alphaproteobacteria (40.6%) and Actinobacteria (6.7%). Of the 271 operational phylogenetic units (OPUs) identified, 13 are likely to represent novel genera and 143 novel species. A comparison with 16S rRNA gene tag data indicated that most of the isolates were rather rare in surface waters, with the exception of five OPUs affiliating with Rhodobacteraceae, Polaribacter, Psychromonas and Pseudoalteromonas. The effort yielded many novel isolates, yet most of the abundant heterotrophic bacteria still remained elusive. The large strain collection obtained will not only provide insights into the succession of the cultivable fraction of the bacterioplankton, but also enable fine-tuned taxonomic and physiological follow-up studies for improving our knowledge on heterotrophic bacteria in North Sea waters.     

Insect wing loss is tightly linked to the treeline: evidence from a diverse stonefly assemblage

The secondary loss of flight in previously winged insect lineages has long fascinated biologists. Habitat stability and isolation are thought to play important roles in driving wing reduction (Roff 1990, 1994), with exposure to high winds suggested to accelerate this process (Darwin 1859), although the role exposure plays in insect wing loss has never been empirically demonstrated. Here we assess fine‐scale distributional records from a diverse regional stonefly assemblage, to demonstrate a widespread association between wing loss and the treeline in New Zealand. The observed pattern suggests that exposure plays a crucial role driving wing loss in alpine insects.     

Cultivation of particle-associated heterotrophic bacteria during a spring phytoplankton bloom in the North Sea

Seawater contains free-living and particle-attached bacteria. Only a small fraction is cultivable on plates. As free-living and particle-associated bacteria differ in their physiological traits, their cultivability on plates may coincide with particle association. Using filtration and Imhoff sedimentation cones, particles were collected during a spring phytoplankton bloom off Helgoland (North Sea) in order to obtain particle-associated bacteria as inocula. Direct dilution plating resulted in 526 strains from 3 µm filtration retentates and 597 strains from settled particles. Motile Gammaproteobacteria from the genera Pseudoalteromonas, Shewanella, Psychrobacter, Vibrio and Colwellia, as well as particle-attached Flavobacteriia affiliating with the genera Tenacibaculum and Gramella, were frequently isolated. As a result, a diverse collection comprised of 266 strains was deposited. Two strains were most likely to represent novel genera and 78 strains were probably novel species. Recently, a high-throughput cultivation study from the same site using seawater as an inoculum had retrieved 271 operational phylogenetic units (OPUs) that represented 88% of the 4136 characterized strains at the species level. A comparison of 16S rRNA gene sequences revealed that the collection obtained matched 104 of the 271 seawater OPUs at the species level and an additional 113 at the genus level. This large overlap indicated a significant contribution of particle-associated bacteria to the cultivable microbiome from seawater. The presence of 49 genera not identified in the larger seawater study suggested that sample fractionation was an efficient strategy to cultivate rare members of the planktonic microbiome. The diverse collection of heterotrophic bacteria retrieved in this study will be a rich source for future studies on the biology of particle-associated bacteria.     

Comparison of phosphorus deficiency indices during a spring phytoplankton bloom in a eutrophic reservoir

1. Phosphorus limitation was studied along the eutrophic, canyon-type ?ímov reservoir (Czech Republic) during a spring phytoplankton bloom. Concentration of soluble reactive phosphorus (SRP), C:P molar ratio in seston, extracellular alkaline phosphatase activity (APA), and P limitation (bioassay) were used as indices for phosphorus deficiency in the phytoplankton. 2. SRP, C:P, APA, and P limitation indicated a moderate P deficiency in the downstream, but not upper, part of the reservoir. 3. Significant correlations between these parameters were found in the downstream part. Chlorophyll a concentration correlated with APA and P limitation in the upper part. 4. APA was significantly enhanced in the phosphorus-deficient phytoplankton. However, APA was apparently not related to total biomass or species composition of the phytoplankton. 5. Generally, APA was closely correlated with pH in the reservoir. However, extracellular alkaline phosphatases, with a pH optimum above 9.0, were induced and active only during the phytoplankton bloom, whereas low background activity of extracellular phosphatases was found at low chlorophyll a concentrations (winter, clear-water phase).